[Python-Dev] Multiple expression eval in compound if statement?

[Skip Montanaro]

I'm horsing around with recognizing switch-like if statements like:

    if x == 1:
        print 1
    elif x == 2:
        print 2
    else:
        print "unknown"

in the compiler and generating O(1) code.  "x" can be any expression, but
must be precisely the same in each elif clause, the comparison operator must
be "==" and the RHS of the test must evaluate to a simple hashable constant
(string, float, integer - still working on None, constant tuples may be
allowed later).  I can currently recognize such constructs and am working on
code generation.

This would represent a semantic change to the Python runtime, because x
would only be evaluated once, whereas in existing usage it can potentially
be evaluated many times.  If evaluating x has side-effects, code like the
above that currently works would break.

In reading the language reference manual, it says:

    It selects exactly one of the suites by evaluating the expressions one
    by one until one is found to be true (see section 5.10 for the
    definition of true and false); then that suite is executed (and no other
    part of the if statement is executed or evaluated). If all expressions
    are false, the suite of the else clause, if present, is executed.

It says nothing about possibly caching values, which is what I'm doing
effectively, but it seems pretty clear that each full test expression will
be evaluated until one evaluates to true.

I can't imagine anyone relying on a side-effect when evaluating x in this
context, and if someone did, their code would be truly fragile.  Still, any
thought as to whether or not such a semantic change to the language might be
acceptable?

Skip